Tuesday, 2 December 2014

There was a time when states of matter were simple: Solid, liquid, gas.
Then came plasma, Bose -Einstein condensate, supercritical fluid and
more. Now the list has grown by one more, with the unexpected discovery
of a new state dubbed “dropletons” that bear some resemblance to liquids
but occur under very different circumstances.

The discovery occurred when a team at the University of Colorado Joint
Institute for Lab Astrophysics were focusing laser light on gallium
arsenide (GaAs) to create excitons.

Excitons are formed when a photon strikes a material, particularly a
semiconductor. If an electron is knocked loose, or excited, it leaves
what is termed an “electron hole” behind. If the forces of other charges
nearby keep the electron close enough to the hole to feel an
attraction, a bound state forms known as an exciton. Excitons are called
quasiparticles because the electrons and holes behave together as if
they were a single particle.

If this all sounds a bit hard to relate to, consider that solar cells
are semiconductors, and the formation of excitons is one possible step
to the production of electricity. A better understanding of how excitons
form and behave could produce ways to harvest sunlight more
efficiently.

Graduate student Andrew Almand-Hunter was forming biexcitons – two
excitons that behave like a molecule, by focusing the laser to a dot
100nm across and leaving it on for shorter and shorter fractions of a
second.

“But the experiment didn’t behave at all in the way we expected,”
Almand-Hunter said. When the pulses were lasting less than 100
millionths of a second exciton density reached a critical threshold. “We
expected to see the energy of the biexcitons increase as the laser
generated more electrons and holes. But, what we saw when we did the
experiment was that the energy actually decreased!”

The team figured that they had created something other than biexcitons,
but were not sure what. They contacted theorists at
Philipps-University, Marburg who suggested they had made droplets of 4, 5
or 6 electrons and holes, and constructed a model of these dropletons'
behavior.

The dropletons are small enough to behave quantum mechanically, but the
electrons and holes are not in pairs, as they would be if the dropleton
was just a group of excitons. Instead they form a “quantum fog” of
electrons and holes that flow around each other and even ripple like a
liquid, rather than existing as discrete pairs. However, unlike liquids
we are familiar with, dropletons a finite size, outside which the
electron/hole association breaks down.

The discovery has been published in Nature.
Perhaps the most remarkable thing is that the dropletons are stable, by
the standards of quantum physics. While they can only survive inside
solid materials, they last around 25 trillionths of a second, which is
actually long enough for scientists to study the way their behavior is
shaped by the environment. At 200nm wide the dropletons are as large as
very small bacteria – a size that can be seen by conventional
microscopes.

"Classical optics can detect only objects that are larger than their
wavelengths, and we are approaching that limit," Mackillo Kira of
Philipps-University who provided much of the theoretical grounding told Scientific American. "It would be really neat to not only detect spectroscopic information about the dropleton, but to really see the dropleton."

JILA lab leader Professor Steven Cundiff says, “Nobody is going to
build a quantum droplet widget." However, the work could help in the
understanding of systems where multiple particles interact quantum
mechanically.

Jenish Patel

Jenish

is a Cisco Certified Internetworking Expert. He is working in the domain of Routing & switching also working with Next Generation Networks implementation.
Apart from that he is actively involved in String Theory Development and Quantum Physics research.